In conventional gas separation using adsorption techniques, it is usual to pass the gaseous mixture into an absorbent bed under pressure and to obtain from the bed an enriched gas mixture which constitutes the desired product and is still at a small positive pressure. If the product is required at a high pressure it must be compressed separately. The adsorbent bed is normally regenerated before it becomes completely saturated by applying a vacuum to the bed which has the effect of drawing off the adsorbed constituents of the gas mixture ready for a further supply to the bed of feed gas mixture to be separated. It will be appreciated that such plants can require three separate pumps in order to produce product gas at high positive pressures and can consequently constitute reasonably complex plants having a relatively high capital cost and power consumption, particularly as the pump required for compressing the feed gas mixture can use a significant amount of power.